Paris prosecutor Francis Molins on Tuesday said the man who killed a police officer and his partner had repeatedly pledged allegiance to the group calling itself the "Islamic State" (IS).

Molins also confirmed that three people had been taken into custody as part of the probe into the double murder on Monday.

The 25-year-old radical Larossi Abballa repeatedly and fatally stabbed the 42-year-old policeman Jean-Baptiste Salvaing outside his home on Monday, before entering the house and slitting the throat of the officer's wife, who also worked for law enforcement authorities. The couple's three-year-old boy was found alive at the end of a three-hour standoff in the suburb of Magnanville, some 35 miles (55 kilometers) west of Paris, at the end of which Abballa was killed.

In 13 minutes of footage filmed at the home of his victims, Abballa repeated his declaration of allegiance to Islamic State leader Abu Bakr al-Baghdadi. "I don't know what to do with him," Abballa said in the video, referring to the couple's son who was sitting behind him on a couch.

The attacker also read a list, found at the scene after he had been killed, of names of public figures he identified as targets.

In negotiations with the elite forces group Raid, Molins said Abballa told them he had sworn allegiance to IS leader Abu Bakr al-Baghdadi three weeks earlier. "He added, I quote, that he had responded to a communiqué from that emir to kill unbelievers in their homes, with their families," said Molins.

The IS-affiliated news outlet al-Amaq cited an unnamed source as saying that an IS fighter had carried out the attack. Agnes Thibault-Lecuivre, spokeswoman at the prosecutor's office, said there was "no reason" to doubt the claim.

Killer was wiretapped

Molins said three people, aged 27, 29 and 44, had been detained in connection with the investigation, but gave no further details. He added that a recent wiretap of Abballa, who was known to police, had given no indication that he was planning an imminent attack.

French President Francois Hollande said supplementary measures might be put in place, and that vigilance should be raised to the highest level. However, the president did not specify further about the actions to be taken. "It is undoubtedly a terrorist act," said Hollande.

"I will stand side-by-side with the security forces of our country as they experience a great sorrow and great anger," the president said.

Abballa came from the French town of Mantes-la-Jolie and is reported to have been convicted of recruiting jihadis to fight in Pakistan in 2013. He was given a two-and-a-half-year prison sentence.

Interior Minister Bernard Cazeneuve on Tuesday visited the police station in Les Mureaux where Salvaing worked.

Fears for all police

Cazeneuve said that more than 100 people seen as potential threats had been arrested in France this year, including in recent weeks.

Following Monday's attack, the interior minister said officers would be allowed to take home their service weapons. "Today every police officer is a target," Yves Lefebvre, from the police union Unite SGP Police-FO, told the AP news agency.

France is on high alert as the Euro 2016 tournament continues, particularly in the wake of the November attacks on Paris which targeted the nation's Stade de France football stadium, as well as bars and cafés in the French capital.

Fears have also risen since the weekend attack that killed 49 people at a gay club in Orlando, Florida, the perpetrator of which claimed to be acting on behalf of IS.

rc/msh (dpa, AFP, AP)